In American usage, a sparkplug lighthouse, sometimes known as a bug light, is a type of caisson lighthouse so named because of its shape. Generally speaking, a sparkplug lighthouse consists of a three-story living area, with the lantern on top; the whole is then placed upon a concrete or metal caisson. The superstructure was usually prefabricated and then lifted onto the caisson by crane. The classic sparkplug light sits in open water, with only the caisson below it and no dry land, but the term is also applied to lights on land or pierheads that are constructed in the same fashion.